This invention addresses corrosion of fasteners connecting equipment such as boat lifts to wooden supports such as pilings. A number of approaches have been used to mitigate this problem, but each has certain disadvantages. This corrosion is not readily observed because it takes place inside an opaque piling, and the visible portions exhibit little or no corrosion. The rate at which the corrosion takes place is generally not appreciated because two different ground voltages are normally involved, and the voltage difference can greatly accelerate the corrosion rate.
For side-mounted boat lift brackets, there are typically two bolts holding each bracket to a piling. The piling, typically pressure treated wood, tends to wick water and exhibit moderate electrical conductivity (hereinafter referred to as quasi-conductivity, as defined in [0021]), and typically is in contact with earth ground potential. A boat lift is normally made of metal, is operated using 120 or 240 volts, and is connected to a personnel safety ground.
Depending on the relative polarity of the grounds, a non-negligible current can flow from the earth ground, preferentially through the lower bolt, through the support bracket, through the boat lift, and to the safety ground. Given sufficient time, this current has been observed to remove the central 85% of the lower bolt, and up to 80% of the cross section of the upper bolt at some locations. The associated voltage was 1 volt, and the current 0.003 amperes. The time to cause the described damage was 0.02 ampere-years.
A partial solution to this problem is to install electrical insulation between the boat lift longitudinal beams and their support brackets. While this is a desirable thing to do, it is inadequate because various other items connected to the personnel safety ground are also connected to various parts of the wooden structure. Such items include flexible metal conduit with an easily penetrated insulating coating, the boat lift control box, and the electrical breaker panel. The result of this was that current from the ocean ground traveled up a piling, transferred to a lower bolt holding a boat lift support bracket, travelled through the bracket, transferred through the upper bolt back into the piling, and continued up the piling to a place that the safety-grounded conduit was in contact with the piling. This caused both bolts to corrode through and for the boat lift to collapse seven years after installation. This collapse caused major property damage and caused a risk of personnel injury or death. The present invention is intended to prevent such an event from occurring.
Other approaches that have been taken to mitigate corrosion in marine environments include use of sacrificial anode material (U.S. Pat. No. 3,887,449-A, Robert B Baer, Jun. 3, 1975; JP-4146994, Untranslated, Sep. 10, 2008), modulated current between anode and cathode (U.S. Pat. No. 5,627,414-A, Fordyce M. Brown et al., May 6, 1997; US-2009138148-A1, Sridhar Deivasigamani, abandoned), and triggerable solid state devices to bring the two ground voltages closer to each other (U.S. Pat. No. 5,840,164-A, Richard E. Staerzl, Nov. 24, 1998). The first of these approaches has the disadvantage that the sacrificial material needs to be monitored and periodically replaced; the second and third have the disadvantage of requiring a power source, special controls, and the disadvantage that the personnel safety ground could be rendered ineffective. Another approach is to use two bolts, one of which is designed to fail long before the second one fails (U.S. Pat. No. 9,823,690, Bowers , et al., Nov. 21, 2017). This approach has the disadvantage that the bolts must be monitored and periodically replaced. In addition to the patent search indicated above, discussions with boat lift manufacturers and an experienced boat lift installer indicate that the prior art provides no satisfactory solution to the problem addressed by this invention.
The inventor believes the present invention has the most overlap with USPC groups 52.515, 52.517, 174.23R, 204.96.16, and 204.196.37, and with CPC groups A47L, B23F, B27K, B41N, C04B, C09D, C09J, C23C, C23F, F16L, G01N, H01B, H01Q, H01R, and H02G.